Most current multi-cell wireless communication systems are Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA) based. In such wireless communication systems, particularly CDMA based systems where terminals are employing differing codes, the terminals often experience and hear intracell interference, as well as other interference from neighboring cells. Herein, interference received from other cells will be referred to as intercell interference or a leakage signal. Intracell interference may be created by a terminal itself and/or by transmissions to or from other terminals in a cell.
OFDMA is a highly flexible multiple-access scheme based on Orthogonal Frequency Division Multiplexing (OFDM) technology. OFDMA is the predominant multiple-access scheme of choice for beyond 3G (B3G) generation broadband wireless systems. The major benefits of OFDMA include simplicity, high scalability, fine granularity, and capacity-achieving performance. In OFDMA, the multiple-access is not only accommodated in the time domain, but also in the frequency domain. OFDMA is similar to conventional Frequency-Division Multiplexing (FDM) in that different terminals occupy different subchannels. The difference lies in the manner in which spectrum is allocated and in how signals are modulated and demodulated. In OFDMA priority is given to minimizing the interference, or crosstalk, among the channels and symbols comprising the data stream. Typically, less importance is placed on perfecting individual channels. OFDMA employs a very broad bandwidth, such as 5 MHz. Each terminal within a cell will use a certain portion of the bandwidth, such as by way of example 10 kHz. An OFDMA band employs numerous narrow frequency bands, referred to as sub-carriers, using Fast Fourier Transform (FFT) techniques. Typical OFDMA systems group a number of the sub-carriers into a subchannel. For example, 64 sub-carriers may be grouped into a subchannel. Within a cell, every terminal will occupy a different set of orthogonal subchannels in a non-overlapping fashion, relative to other terminals in the cell. As a result there is little or no intracell interference in OFDMA. This is a great advantage of OFDMA. Terminals have a clean channel and can transmit, using adaptive modulations, as fast as possible based on the terminal's subchannel's Received Signal Strength Indication (RSSI) and/or the like.
However, while there is no intracell interference in multi-cell systems employing OFDMA, a terminal will hear interference from other cells using the same subchannel as the terminal. As note above such interference may be referred to as intercell interference or a leakage signal. Typically, existing systems have used one of two categories of solutions in an attempt to address this problem. One is to use sectorization, as in a Global System for Mobile Communications (GSM) system, OFDMA based systems may try to reduce or at least suppress intercell interference by orienting the different antenna patterns and/or by using different frequencies, for sectors from two cells pointing to each other. However, this interference mitigation technique reduces overall system capacity, in that the subchannels that can be used in particular areas of a cell at any particular time is typically reduced.
The second approach for mitigating intrercell interference in typical OFDMA systems has been to employ subchannel patterns which “hop” over time. This subchannel may hop across the sub-carriers of the 5 Mhz bandwidth of the OFDMA band, with hopping patterns in different cells of a system being different. In this way a certain degree of so-called interference averaging is achieved. In effect, occasionally a “direct hit,” for terminals from different cells using a same sub-carrier, may be encountered. However, in a next time slot, because the different cells use a different hopping pattern, a direct hit is avoided. The randomized, “smoothed” interference pattern resulting may be treated like other background interference. However, a best case situation, where there are no hits at all, will not typically result. Therefore, a problem exists with the latter prior intercell interference mitigation technique for multi-cell OFDMA systems in that by eliminating the worst case scenario of sequential direct hits, the best case scenario of no hits is not available. Some OFDMA systems using an interference averaging approach to deal with the intercell interference manage the interference by sometimes combining interference together, and sometimes separating the interference. Generally, interference averaging improves the worst-case performance of the system at the expense of lower network capacity.